"Please check device readiness".
"Please check device state". [Then clarify: {on/off} / {online/offline} /
{enabled/disabled} ].
However, I'm not sure that these wordings represent much of an improvement with respect to your options.
One problem in this particular context is the ambiguity of the instruction: it is unclear what should happen after the readiness of the device has been checked. For instance, if it is found to be off, should it then be turned on? Alternatively, if it is found to be on, should it be turned off?
I'd therefore be inclined to adapt the wording of your example accordingly. For instance:
Please ensure the device is enabled.
Make sure the device is turned on.
Ensure the device is online.
Verify that the device is connected to the server.
If the device is on, switch it off.